This invention relates to liquid level measurement, particularly in tanks, sumps and bilges.
Liquid level sensors are used in liquid containment tanks and sumps to operate pumps that control the level of liquid in the tank or sump. They are also used to provide an alarm if the liquid reaches certain critical levels. Liquid level sensors are used in:
Septic Tanks PA1 Industrial Liquids Storage Tanks PA1 Marine Bilge Pumping Systems PA1 Sump Pumping Systems PA1 Potable Water Storage Tanks PA1 "Level Measurement: In septic tanks, floats control the pumps that regulate the flow of water. The floats are a weak link in the system and often fail. We need an alternative technology to measure the liquid level in the septic tank and to control the pumps when the water reaches the target level. The liquid has a nearly neutral pH, but contains foam, suspended solids and biological solids, so the measurement technology must tolerate these interferences as well as be reasonably economical. Jack Firkins, Product Manager, Orenco Systems Inc., Sutherlin, Oreg."
Many of the presently-employed liquid level sensors use a moving float to activate electromechanical switches which control pump motors. In some of these devices the float is hinged, in others it is constrained to move up and down inside of a chamber that allows entry of the liquid whose level is to be measured. The float mechanisms are the least reliable component in these systems. The following quote is from Photonics Magazine, November 1999 Issue, Page 76:
Examples of hinged float devices are to be found in:
Leistiko U.S. Pat. No. 3,662,131 Niedermeyer U.S. Pat. No. 4,086,457 Van Fossen U.S. Pat. No. 5,250,768 Hutchinson U.S. Pat. No. 5,939,688 Pottharst U.S. Pat. No. 3,686,451
Some disadvantages of the devices described by Leistiko, Niedermeyer and Van Fossen are that they require a lot of space for the rotation of the float about its hinge and that they cannot measure small changes in liquid level. In addition, Leistiko and Niedermeyer use mercury switches, which are a potential hazard in the event of failure, when used in potable water systems.
Van Fossen describes a complex mechanical switch with pivots, cams and other moving parts that are subject to wear and failure.
Both Hutchinson and Poftharst describe a pivot arm of a float that penetrates the wall of the liquid containment vessel, which is not practical in many types of installations, buried tanks being one example. The required resilient seal at the penetration is difficult to fabricate and prone to failure.
Liquid level sensors that use moving floats constrained in or by chambers are described in:
 Ganderton U.S. Pat. No. 4,052,900 Ganzon, et. al. U.S. Pat. No. 5,562,422 Bardoorian U.S. Pat. No. 4,836,632
In both Ganderton and Ganzon, solids suspended in the liquid can interfere with the movement of the float in the float chamber, causing the devices to fail. In Bardoorian, the float slides on the outside of a chamber, but is also susceptible to interference by suspended solids. Bardoorian also describes a complex magnetic linkage between a magnet on the float on the outside of the chamber and a mirror attached to a magnet on the inside of the chamber.
Wachter, U.S. Pat. No. 4,483,192, describes a displacer attached to the unsupported end of a cantilever beam. Buoyant force acting on the displacer causes bending of the beam. The beam is instrumented with one or more strain gauges. A strain gauge-instrumented beam is expensive to fabricate. It is necessary but difficult to electrically and chemically insulate the external strain gauge(s) from the liquid being measured. Strain gauges require sophisticated, expensive electronics to create reliable binary (switching) signals from their low-amplitude analog signal output.
Examples of liquid level sensors that use differential pressure-responsive diaphragms are:
 Alm U.S. Pat. No. 3,939,383 Siegel U.S. Pat. No. 4,019,387 Glover, et. al. U.S. Pat. No. 4,843,883
A disadvantage of this type of liquid level sensor is that the diaphragms used have to be excessively large to activate switches, in the typical application where the liquid level changes are not large. All three of these implementations require complex mechanical linkages to produce a switching action.
Still another type of liquid level sensor measures the change in apparent weight of a displacer suspended in the liquid containment tank. Examples of this type are given in:
 Holm U.S. Pat. No. 4,843,876 Spitzer U.S. Pat. No. 4,875,370 Crawford, et. al. U.S. Pat. No. 5,132,923
A disadvantage of all three of these implementations is that the displacer must extend throughout the vertical range of levels that it is desired to measure, making the displacer quite large in many cases. An additional disadvantage in Holm and Crawford is the requirement for a complex transducer for weighing the displacer (from which the displacer is suspended) and the associated expensive electronics to calibrate the transducer and convert its output to a switch control signal. In Spitzer, the suspension system for the displacer is an expensive and complex conditionally-stable closed-loop feedback control system (a servomechanism), requiring an electromagnet capable of suspending the displacer and a position pickoff to determine its position.
Still another type of liquid level sensor relies on measurement of the propagation of optical energy through the liquid. Examples of this type of sensor are found in:
 Hansel, et. al. U.S. Pat. No. 4,069,838 Rait U.S. Pat. No. 4,075,616 Finney, et. al. U.S. Pat. No. 5,065,037
Hansel relies on a gap in an optical path being filled by the liquid to be measured, the liquid altering the optical transmission characteristics of the path. A disadvantage of this approach is that suspended solids can precipitate out of the liquid over time, changing the optical transmission characteristics of the path.
Rait relies on the presence of the liquid interfering with the detection of light from an optical source. Again, suspended solids can precipitate out onto the optical surfaces, interfering with the detection process whether the liquid is present or not.
In Finney, optical measurements are made by passing light through a chamber which is filled with the liquid whose level is to be measured. Again, solids precipitating onto the chamber windows can cause serious error in the measurements.
All three of the above implementations require relatively complex and expensive electronics to create a reliable switching signal from the low-amplitude analog outputs of the optical detectors.
Still another type of liquid level sensor utilizes measurement of the propagation-time of acoustic or ultrasonic energy. Typically, the energy is caused to reflect from the liquid surface and the time from transmission of the energy to reception of the echo is measured, as in Sonar systems. Examples of this technique are:
 Soltz U.S. Pat. No. 4,821,569 Olson, et. al. U.S. Pat. No. 4,901,245
These systems tend to be expensive due to the requirement for high-power ultrasonic transmitters and complex signal processing hardware and software to differentiate between the desired echo and spurious echos from other parts of the tank. Their accuracy varies as a function of the shape and size of the tank, the liquid level and the presence of structures in the tank. Often they have to be calibrated to the specific tank.
The above-described prior art liquid level sensors are generally overly complex, unreliable, unsuited to the conditions found in many liquid level measurement applications, require excessive space for installation or are expensive to manufacture. What is needed therefore is a simple, low-cost and small liquid level sensor that can function reliably in liquid environments that cause problems for prior art devices.